Internal combustion engines benefit from real time estimation of combustion quality and start of combustion (SOC). Knowing when combustion commences can help provide a control strategy that adjusts the SOC in future engine cycles to allow for improved performance of the engine. One engine that benefits substantially from SOC monitoring or estimation, is an engine that relies on the auto-ignition of a pre-mixed fuel and air charge. The auto-ignition of a pre-mixed fuel and air charge is referred to here as pre-mixed charge compression ignition, or PCCI. There tends to be emissions and efficiency benefits with PCCI combustion modes over typical diffusion or flame propagation combustion modes. An additional combustion strategy that benefits from SOC estimation is an engine that supplements the energy provided by a PCCI combustion event with a directly injected quantity of fuel generally provided once SOC has commenced. This type of engine is known as a premixed charge direct injection (PCDI) engine.
In general, various fuels or mixtures of fuels can be used to drive a PCCI or PCDI engine. By way of example, gaseous-fuelled high compression ratio engines can operate effectively where a small amount of pilot fuel is introduced into a pre-mixed charge containing gaseous fuel early in the compression stroke of an engine. The pilot fuel changes the auto-ignition properties of the premixed gaseous fuel. The injection timing and quantity of the pilot fuel can be manipulated such that the charge auto-ignites when the piston is at or near top dead center. However, the pilot fuel timing and quantity needed to auto-ignite the charge at the desired time of the engine cycle depends on many variables that can change over time and between cylinders. For example, one cylinder may run hotter than others (due to less cooling through the cylinder walls, or more trapped residual gases), thereby needing a smaller pilot fuel quantity to auto-ignite the charge at the desired time as compared to the other cylinders. Similarly, variations in fuel quality or intake manifold temperature over time forces the pilot fuel quantity and timing to be adjusted to maintain a fixed SOC. Preferably, an accurate estimate of the SOC in each cylinder is used in a feedback control loop, where a control lever such as pilot fuel timing or quantity is used to adjust the SOC to a target value. In this way, the PCCI/PCDI application is run in an efficient and robust manner.
Currently, SOC control is generally provided by algorithms that rely on a direct measurement of a signal indicative of pressure within the combustion chamber. For example, U.S. Pat. No. 6,598,468 and German Patent 4341796.5 use techniques to correlate a measured parameter indicative of pressure to SOC. The estimated SOC value is then used to adjust levers available to the controller to influence the SOC in future engine cycles. That is, a feedback control loop is used to minimize the error between the measured SOC and a target SOC. An operator specifies the target SOC. A sensor that measures the deflection of a diaphragm in contact with the in-cylinder pressure using optical methods is an example of such a pressure sensor. An optical sensor of this type could provide sufficiently accurate pressure measurements from the combustion chamber for the techniques taught in the art. Optical sensors, or other direct pressure measurement instruments, can be expensive and may lack the reliability and robustness (due to the harsh environment within a combustion chamber) required for the application.
An alternate sensor that can be used to estimate pressure in the combustion chamber is an accelerometer. The techniques taught above are used to estimate an SOC from the measured accelerometer data: see U.S. Pat. No. 6,408,819 and Lyon, “Cepstral analysis as a tool for robust processing, deverberation, and detection of transients”, Mechanical Systems and Signal Processing, (Academic Press: 1992), 6(1), p 1–15. Such techniques are valuable, as accelerometers tend to be less expensive, and currently more reliable and more robust than direct pressure measuring sensors. However, the drawback with U.S. Pat. No. 6,408,819 is that this technique relies on a method of reconstructing a pressure signal that is unlikely to be sufficiently accurate. Cepstral filtering taught by Lyon can also be used to provide a pressure signal for use with U.S. Pat. No. 6,408,819. However, the cepstral filtering taught by Lyon reconstructs the combustion pressure. It may be possible to develop a method to extract parameters from the combustion pressure that would correlate with SOC. However, one of these parameters is magnitude. Where cepstral analysis is used, the magnitude varies in a non-linear fashion with load. A standard reconstruction technique results in a linear variation in magnitude with respect to load; see DE 43 41 796.5. The non-linearity introduced by cepstral filtering creates complexities when correlating parameters to SOC. The result can be higher errors in the estimated SOC for a given cycle of the engine. The combustion control technique taught in U.S. Pat. No. 6,598,468 relies on a measure of the in-cylinder pressure prior to the on-set of combustion, and a second measure after the combustion has begun. Relying on a standard pressure reconstruction technique would result in a relatively inaccurate picture of the in-cylinder pressure prior to combustion. Cepstral filtering improves the reliability of the measured pressure, but in doing so provides a measure of the combustion pressure. That is, the use of an accelerometer may not give an accurate measure of the in-cylinder pressure prior to combustion, and hence can lead to problems in applying the technique proposed in U.S. Pat. No. 6,598,468.
The applicant has addressed these problems by developing a method and apparatus using accelerometer data that delivers an SOC estimate that is comparable to estimates provided by directly measured pressure indicative signals.